a check box on the alternate settings screen to match alt theme to main theme once checked if the main theme is updated the alt them should change as well 

we use the main channel for our overhead projectors and the alt for our livestream which is run from a different computer using ndi

This has been requested many times since Alternative Output was introduced in Windows 7. You have to re-create the theme each time you edit the presentation and you can only use one theme during each schedule. Previous posts have been archived on this subject over the years.